make 'admin' a safe user name

make 'admin' a safe user name

'admin' is a pretty common username that people try when installing;
it was blacklisted because all of our admin panels were at /admin/*,
which would conflict with the admin user's namespace.

Changed the location of all admin panels to /panel/*, blacklisted the
nickname 'panel', and allowed 'admin'. Tested with a fresh install;
seems to work great.